This invention relates to pumping systems and more particularly to pumping systems that utilize a pulsating pump to draw samples from a source of liquid.
It is known to pump liquids from a liquid source through a pulsating pump, such as for example a peristaltic pump, from Douglas M. Grant U.S. Pat. No. 4,415,011, issued Nov. 15, 1983, and from Carl D. Griffith U.S. Pat. No. 4,660,607, issued Apr. 28, 1987. In such a process, the water interface in the conduit through which the liquid is being pumped is sensed to provide an indication of where the liquid is in the conduit.
Several different sensing mechanisms have been utilized in such pumps such as an optical sensing mechanism, a capacitance sensing mechanism and a electrical conductivity sensing mechanism. The information about the sensed interface is utilized together with other information to meter a fixed volume of liquid into one or more sample containers. U.S. Pat. No. 4,415,011 discloses the metering of liquid by counting cycles of the pump from the shaft of the pump.
In the prior art apparatus, the sensors are either internal or external to the conduit and utilize several different arrangements such as by sensing a change in capacitance between two electrodes outside the conduit as the liquid interface passes through or by sensing changes in the absorption of light transmitted through the conduit or changes in electrical conductivity.
These prior art pumps and sensing mechanisms have several disadvantages such as for example: (1) under some circumstances, the sensing mechanism may have difficulty in distinguishing between a continuous flow of the liquid and spurts of liquid that may be splashed through the sensing point; (2) the pump may slow due to battery drain or other unexpected effects; (3) the head of water may suddenly change, causing variations in pumping; or (4) conductivity and capacitive sensors are prone to malfunctions caused by bridges and changes in the conductivity of liquids.